Highly cross-linked polymers have been studied widely as matrices for composites, foamed structures, structural adhesives, insulators for electronic packaging, etc. The densely cross-linked structures are the basis of superior mechanical properties such as high modulus, high fracture strength, and solvent resistance. However, these materials are irreversibly damaged by high stresses due to the formation and propagation of cracks. Polymerization stress is originated from polymerization shrinkage in combination with the limited chain mobility. Which eventually leads to contraction stress concentration and gradually such a trapped stress would release and caused microscopically the damage in certain weak zone like interfacial areas. Macroscopically it is reflected as debonding, cracking, or the like. Similarly, the origin of the contraction stress in current adhesive restorations is also attributed to the restrained shrinkage while a resin composite is curing, which is also highly dependent on the configuration of the restoration. Furthermore, non-homogeneous deformations during functional loading can damage the interface as well as the coherence of the material. Various approaches have been explored by limiting the overall stress generation either from the restorative materials, or by minimizing a direct stress concentration at the restored interface. It included, for example, new resin, new resin chemistry, new filler, new curing process, new bonding agent, and even new procedure.
There has been much attention paid to new resin matrix development that could offer low polymerization shrinkage and shrinkage stress. For example, various structure and geometry derivatives of (meth)acrylate-based resin systems; non-(meth)acrylates resin systems, non-radical-based resin system. In addition, for light curable, low shrink dental composites, not only new resin systems and new photoinitiators, new filler and filter's surface modification have also been extensively explored, such as filler with various particle size and size distribution, from nanometer to micrometer, different shape, irregular as milled or spherical as-made. It can also be different in composition like inorganic, organic, hybrid. Although an incremental improvement has been achieved with each approach and/or their mutual contribution, polymerization stress is still the biggest challenge in cured network systems.
Dental composite is formulated by using organic or hybrid resin matrix, inorganic or hybrid fillers, and some other ingredients such as initiator, stabilizer, pigments, etc., so as to provide the necessary esthetic, physical and mechanical property for tooth restoration. It is well known that polymerization shrinkage from cured dental composite is one of dental clinicians' main concerns when placing direct, posterior, resin-based composite restorations. Although there are evolving improvements associated with resin-based composite materials, dental adhesives, filling techniques and light curing have improved their predictability, the shrinkage problems remain. In fact, it is the stress associated to polymerization shrinkage that threatens marginal integrity and lead to marginal gap formation and microleakage, which may contribute to marginal staining, post-operative sensitivity, secondary caries, and pulpal pathology.
A common approach to reduce the polymerization shrinkage of dental composite is to increase the filler loading, especially for posterior restoration. However, the higher viscosity of these highly filled composites may not adapt as well to cavity preparations. It has been demonstrated that to initially place a flowable composites which, with less filler content, have greater flexibility, could reduce microleakage than direct application of microhybrid and packable composite restorations, but this benefit may be offset by the increasing polymerization shrinkage for the flowable composite itself. Therefore, it is also highly desirable to develop low shrinkage, especially low curing stress flowable composite, in order to really reduce microleakage as mentioned above.
The challenge in developing any dental composite is to balance the overall performance, including esthetic appearance, handling character as well, in addition to low curing stress and necessary mechanical strength. Unfortunately, superior mechanical strength usually is a result of increasing cross-linking density, from which an unwanted polymerization shrinkage and shrinkage stress always accompany. There is increasing effort to develop new resin systems in the attempt to minimize such shrinkage and stress accordingly. For example, reducing the polymerizable groups in the resin matrix by designing resin monomer with different size and shape indeed work well to some extent in this regard. However, it is usually resulted in decreasing mechanical strength and losing certain handling characteristic because of the limited molecular chain mobility and the limited polymerization conversion. In addition the shrinkage can also be reduced by using special filters which allow an increase in filler loading without compromising too much in handling property. Even so, the curing stress from most of flowable composites remains substantially high. Obviously, it is highly desirable to develop flowable dental composition with low curing stress.